In the manufacture of integrated circuits, various processes are required to be performed on the silicon wafer that is the substrate of the chips. During manufacture, the wafer is transformed from a circular substrate of silicon to a finished wafer containing multiple chips. On of the elements of this manufacturing process is the transfer of the wafers between various carriers, termed boats, so that different operations may be performed on the wafers.
The wafer transfer operation is one of the factors that significantly affects chip yield. Therefore, a great deal of effort has been expended to improve the methods of wafer transfer.
Wafers are generally transported in a side-by-side, vertical orientation. One of the disadvantages of the prior art is that there is generally a fairly large variance in the repeatability of the transfer motion. That is, the machine performing the transfer will at first move the wafers to the proper position, but after some repetitions, will fall out of adjustment. When the tolerance of a given operation is exceeded, the wafers are brought into contact with a foreign object. This unwanted contact can damage chips, and lower the yield of the process.
Another problem inherent with some of the prior art machines is that moving parts can create particulate contamination. Wafer fabrication is performed in a "clean room" environment. Thus, any factor that generates contamination is a detrimental factor to yield.
Still another problem in the prior art is the maintenance of spacing between the individual wafers in a boat. If the spacing is not maintained, the wafers will contact each other, and again reduce yield.